1. Field of the Invention
This invention relates to the field of heat exchangers, and in particular to a rotary regenerative heat exchanger having a particular construction of radial and/or circumferential seals.
2. Prior Art
In a conventional regenerative heat exchanging apparatus, a hot gas conduit, for example the exhaust of a fuel burning power plant, is routed alongside a cool gas conduit, for example the combustion air inlet to the plant. At one or more points along the path of the conduits, a movable heat exchanging body is disposed such that the hot gases can be passed through the heat exchanging body, which collects heat. The heat exchanging body is moved, for example by rotating it, such that the cool inlet gas is passed through heated portions of the body to extract the heat which was collected. Typically, the heat exchanging body is a cylindrical arrangement of sheet material, for example lengths of corrugated sheet metal wrapped in a roll about an axis around which the cylinder is to be rotated. The cylinder is typically subdivided into angular sectors by means of radial plates extending the axial length of the cylindrical body such that the gases passing through the cylindrical body are confined to one radial sector and cannot leak between the hot and cool sides. Similarly, structures defining the hot gas passage through the stationary housing in which the cylinder turns, and also the cool gas passage, also define angular sectors. When an angular sector as defined by plates of the cylindrical heat exchanging body aligns with the angular sector of the hot or cool passage, respectively, then gases pass through that sector.
The radially-extending plates of the heat exchange body are frequently sealed at the axial ends of the heat exchange body to the housing in which the heat exchange body turns. In this manner, intake air cannot leak from the air side to the gas side by passing around the outside of the heat exchanging body. There are numerous examples of different types of seals in the prior art, including, for example U.S. Pat. No. 4,098,323-Wiegard et al, disclosing axially-extending circumferential seals. U.S. Pat. No. 3,011,766-Hess discloses inner and outer radially-directed seals in a toroidal heat exchanger.
Cylindrical body heat exchangers with seals bridging from cylindrical bodies to their housings are disclosed for example, in U.S. Pat. Nos. 4,383,573-Wincze; 3,822,739-Kurschner; 3,545,532-Waitkus; 3,380,514-Eisenstein; 3,216,486-Hall et al; 3,182,715-Rayburn; 2,945,681-Burchfield; 2,692,760-Flurschutz; 2,549,656-Yerrick et al; 1,522,825-Ljungstrom; 1,746,598-Ljungstrom; 2,287,777-Boestad; 2,517,512-Tigges et al; 4,673,026-Hagar et al; 2,549,583-Eckersley; and 4,791,980-Hagar et al. The disclosures of all said patents are hereby incorporated.
Substantial difficulties are encountered in attempting to adequately seal between the cylindrical heat exchange body and the housing in which it rotates. The environment of combustion exhaust gases is corrosive and abrasive. Seals which bear in this environment against the cylindrical body from the housing, or vice versa, tend to be worn down quickly. Another problem is encountered in that heating and cooling of the heat exchange body produce thermal stresses causing the heat exchange body to warp or sag with variations in temperature, and affecting the clearance between the body and the housing, i.e., the dimensions of the space to be bridged by the seals Where, for example, a heat exchange body is expected to rotate on a vertical axis deformation of the cylindrical heat exchange body with heating may take the form of axial displacement at the circumference of the heat exchange body, the heat exchange body sagging to resemble the form of a truncated cone. Of course, should the seals be dimensioned to accommodate a precisely cylindrical arrangement, the lower seals will be quickly worn down and the upper seals may define a gap with respect to the top of the housing.
The foregoing prior art references attempt a number of seal arrangements in an effort to accommodate these very demanding conditions. Spring biased displaceable seal elements, magnetically displaceable seal elements, telescopically displaceable seal elements and other variations all appear in the prior art. However, more and more complicated mounting and/or displacement configurations for the seals, cause greater and greater problems due the abrasive nature of the environment interfering with correct operation of the seals. In such an abrasive and corrosive environment, simpler seals may be longer lived. On the other hand, simpler seals are more difficult to adapt to the variations in the span of the seal which are required to maintain a seal.
Where a longer lived seal is desired, it may seem appropriate to provide a heavier seal body such that abrasion will take longer to wear away the seal in the required dimension. A thicker seal, however, is likely to be even less flexible and therefore may be less able to accommodate dimensional variations which occur at operating temperatures.
A further problem encountered in the prior art relates to the replacement of worn seals. Any seal will eventually deteriorate and must be replaced. When refurbishing the seal, it is necessary to determine precisely where the radial and/or circumferential seals to be replaced should be optionally mounted with respect to the housing. It is normally not possible for maintenance personnel to access the heat exchanger in its operative condition, when high temperatures and fumes are present. As a result, it is necessary to estimate the correct position of the seals when replacing the wear surfaces thereof. The seals are inherently flexible and displaceable, and as a result it is most difficult according to the prior art to correctly position the seals. Too often, seals are inadvertently set at a wrong clearance, the seals being quickly worn off and/or the efficiency of the heat exchanger being lost by widely gapped seals.
Regenerative heat exchangers have been effective to conserve energy by usefully extracting excess heat. As a practical matter, however, a number of problems remain which make it difficult or impossible according to the prior art to enjoy the full potential efficiency of a regenerative heat exchanger. The present invention solves a number of the practical problems associated with sealing the heat exchanger, including problems with respect to wear, flexibility, replacement and correct mounting of the radial and circumferential seals.